1. Introduction
Several iSCSI implementations have been built since [RFC3720] was
published and the iSCSI community is now richer by the resulting
implementation expertise. The goal of this document is to leverage
this expertise both to offer clarifications to the [RFC3720]
semantics and to address defects in [RFC3720] as appropriate. This
document intends to offer critical guidance to implementers with
regard to non-obvious iSCSI implementation aspects so as to improve
interoperability and accelerate iSCSI adoption. This document,
however, does not purport to be an all-encompassing iSCSI how-to
guide for implementers, nor a complete revision of [RFC3720].
Instead, this document is intended as a companion document to
[RFC3720] for the iSCSI implementers.
iSCSI implementers are required to reference [RFC3722] and [RFC3723]
in addition to [RFC3720] for mandatory requirements. In addition,
[RFC3721] also contains useful information for iSCSI implementers.
The text in this document, however, updates and supersedes the text
in [RFC3720] whenever there is such a question.
2. Definitions, Acronyms, and Document Summary
2.1. Definitions
I/O Buffer
A buffer that is used in a SCSI Read or Write operation so SCSI
data may be sent from or received into that buffer. For a read or
write data transfer to take place for a task, an I/O Buffer is
required on the initiator and at least one is required on the
target.
SCSI-Presented Data Transfer Length (SPDTL)
SPDTL is the aggregate data length of the data that the SCSI layer
logically "presents" to the iSCSI layer for a Data-In or Data-Out
transfer in the context of a SCSI task. For a bidirectional task,
there are two SPDTL values -- one for Data-In and one for Data-
Out. Note that the notion of "presenting" includes immediate data
per the data transfer model in [SAM2], and excludes overlapping
data transfers, if any, requested by the SCSI layer.
Third-party
A term used in this document to denote nexus objects (I_T or
I_T_L) and iSCSI sessions that reap the side effects of actions
that take place in the context of a separate iSCSI session, while
being third parties to the action that caused the side effects.
One example of a third-party session is an iSCSI session hosting

SN Sequence Number
SNACK Selective Negative Acknowledgment - also
Sequence Number Acknowledgement for data
TCP Transmission Control Protocol
TMF Task Management Function
TTT Target Transfer Tag
UA Unit Attention
2.3. Clarifications, Changes, and New Semantics
This document specifies certain changes to [RFC3720] semantics as
well as defines new iSCSI semantics. In addition, this document also
clarifies the [RFC3720] semantics. This section summarizes the
contents of the document, categorizing each section into one or more
of a clarification, change, or new semantic.
Section 3.1.1: Clarification on iSCSI residuals computation
general principles
Section 3.1.2: Clarification on iSCSI residuals computation with
an example
Section 3.2: Clarification on R2T ordering requirements
Section 3.3: New Semantics for Response Ordering in multi-
connection iSCSI sessions
Section 4.1.2: Clarifications, changes, and new semantics on
multi-task abort semantics that all implementations must comply
with
Section 4.1.3: Changes and new semantics (FastAbort semantics) on
multi-task abort semantics that implementations should use for
faster error recovery
Section 4.1.3.1: Changes in iSCSI clearing effects semantics
resulting from new FastAbort semantics
Section 4.1.4: New Semantics on third-party session interactions
with the new FastAbort semantics

Section 4.1.5: Clarification on implementation considerations
related to outstanding data transfers in order to realize correct
iSCSI protocol behavior
Section 4.1.6: Clarification on the intent behind FastAbort
semantics (not clarifications to [RFC3720] semantics)
Section 5.1: Clarification on error recovery semantics as
applicable to Discovery sessions
Section 5.2.1: Clarification and new semantics on applying the
Initiator Session Identifier (ISID) RULE ([RFC3720]) to Unnamed
Discovery Sessions
Section 5.2.2: Clarification on applying the ISID RULE to Named
Discovery Sessions
Section 5.3: Clarification on allowed PDU types and target Logout
notification behavior on a Discovery session
Section 6.1: Clarification on the legality of the Target Portal
Group Tag (TPGT) value of zero
Section 6.2: Clarification on the negotiating order of SessionType
with respect to other keys
Section 6.3: Clarification on the NotUnderstood negotiation
response on declarative keys and the implied semantics
Section 6.4: Clarification on the number of legal outstanding
negotiation PDUs (Text or Login-related)
Section 7.1: Clarification on usage of the ITT value of 0xffffffff
Section 7.2: Clarification on what constitutes format errors for
the purpose of error recovery defined in [RFC3720]
Section 7.3: Change in error recovery semantics for the case of
discarding unsolicited PDUs
Section 7.4: Clarification on the intended level of error checking
on inbound PDUs
Section 8.1: New semantics for a new AsyncEvent code
Section 8.2: Change of legal status for Reject reason code 0x0b;
it is now deprecated

Section 9.1: New semantics for a new text key TaskReporting
3. iSCSI Semantics for SCSI Tasks
3.1. Residual Handling
Section 10.4.1 of [RFC3720] defines the notion of "residuals" and
specifies how the residual information should be encoded into the
SCSI Response PDU in the Counts and Flags fields. Section 3.1.1
clarifies the intent of [RFC3720] and explains the general
principles. Section 3.1.2 describes the residual handling in the
REPORT LUNS scenario.
3.1.1. Overview
SCSI-Presented Data Transfer Length (SPDTL) is the term this document
uses (see Section 1.1 for definition) to represent the aggregate data
length that the target SCSI layer attempts to transfer using the
local iSCSI layer for a task. Expected Data Transfer Length (EDTL)
is the iSCSI term that represents the length of data that the iSCSI
layer expects to transfer for a task. EDTL is specified in the SCSI
Command PDU.
When SPDTL = EDTL for a task, the target iSCSI layer completes the
task with no residuals. Whenever SPDTL differs from EDTL for a task,
that task is said to have a residual.
If SPDTL > EDTL for a task, iSCSI Overflow MUST be signaled in the
SCSI Response PDU as specified in [RFC3720]. The Residual Count MUST
be set to the numerical value of (SPDTL - EDTL).
If SPDTL < EDTL for a task, iSCSI Underflow MUST be signaled in the
SCSI Response PDU as specified in [RFC3720]. The Residual Count MUST
be set to the numerical value of (EDTL - SPDTL).
Note that the Overflow and Underflow scenarios are independent of
Data-In and Data-Out. Either scenario is logically possible in
either direction of data transfer.
3.1.2. SCSI REPORT LUNS and Residual Overflow
This section discusses the residual overflow issues citing the
example of the SCSI REPORT LUNS command. Note however that there are
several SCSI commands (e.g., INQUIRY) with ALLOCATION LENGTH fields
following the same underlying rules. The semantics in the rest of
the section apply to all such SCSI commands.

The specification of the SCSI REPORT LUNS command requires that the
SCSI target limit the amount of data transferred to a maximum size
(ALLOCATION LENGTH) provided by the initiator in the REPORT LUNS CDB.
If the Expected Data Transfer Length (EDTL) in the iSCSI header of
the SCSI Command PDU for a REPORT LUNS command is set to at least as
large as that ALLOCATION LENGTH, the SCSI layer truncation prevents
an iSCSI Residual Overflow from occurring. A SCSI initiator can
detect that such truncation has occurred via other information at the
SCSI layer. The rest of the section elaborates this required
behavior.
iSCSI uses the (O) bit (bit 5) in the Flags field of the SCSI
Response and the last SCSI Data-In PDUs to indicate that an iSCSI
target was unable to transfer all of the SCSI data for a command to
the initiator because the amount of data to be transferred exceeded
the EDTL in the corresponding SCSI Command PDU (see Section 10.4.1 of
[RFC3720]).
The SCSI REPORT LUNS command requests a target SCSI layer to return a
logical unit inventory (LUN list) to the initiator SCSI layer (see
Section 6.21 of SPC-3 [SPC3]). The size of this LUN list may not be
known to the initiator SCSI layer when it issues the REPORT LUNS
command; to avoid transferring more LUN list data than the initiator
is prepared for, the REPORT LUNS CDB contains an ALLOCATION LENGTH
field to specify the maximum amount of data to be transferred to the
initiator for this command. If the initiator SCSI layer has under-
estimated the number of logical units at the target, it is possible
that the complete logical unit inventory does not fit in the
specified ALLOCATION LENGTH. In this situation, Section 4.3.3.6 in
[SPC3] requires that the target SCSI layer "shall terminate transfers
to the Data-In Buffer" when the number of bytes specified by the
ALLOCATION LENGTH field have been transferred.
Therefore, in response to a REPORT LUNS command, the SCSI layer at
the target presents at most ALLOCATION LENGTH bytes of data (logical
unit inventory) to iSCSI for transfer to the initiator. For a REPORT
LUNS command, if the iSCSI EDTL is at least as large as the
ALLOCATION LENGTH, the SCSI truncation ensures that the EDTL will
accommodate all of the data to be transferred. If all of the logical
unit inventory data presented to the iSCSI layer -- i.e., the data
remaining after any SCSI truncation -- is transferred to the
initiator by the iSCSI layer, an iSCSI Residual Overflow has not
occurred and the iSCSI (O) bit MUST NOT be set in the SCSI Response
or final SCSI Data-Out PDU. This is not a new requirement but is
already required by the combination of [RFC3720] with the
specification of the REPORT LUNS command in [SPC3]. However, if the
iSCSI EDTL is larger than the ALLOCATION LENGTH in this scenario,
note that the iSCSI Underflow MUST be signaled in the SCSI Response

PDU. An iSCSI Underflow MUST also be signaled when the iSCSI EDTL is
equal to the ALLOCATION LENGTH but the logical unit inventory data
presented to the iSCSI layer is smaller than the ALLOCATION LENGTH.
The LUN LIST LENGTH field in the logical unit inventory (the first
field in the inventory) is not affected by truncation of the
inventory to fit in ALLOCATION LENGTH; this enables a SCSI initiator
to determine that the received inventory is incomplete by noticing
that the LUN LIST LENGTH in the inventory is larger than the
ALLOCATION LENGTH that was sent in the REPORT LUNS CDB. A common
initiator behavior in this situation is to re-issue the REPORT LUNS
command with a larger ALLOCATION LENGTH.
3.2. R2T Ordering
Section 10.8 in [RFC3720] says the following:
The target may send several R2T PDUs. It, therefore, can have a
number of pending data transfers. The number of outstanding R2T
PDUs is limited by the value of the negotiated key
MaxOutstandingR2T. Within a connection, outstanding R2Ts MUST be
fulfilled by the initiator in the order in which they were
received.
The quoted [RFC3720] text was unclear on the scope of applicability
-- either per task, or across all tasks on a connection -- and may be
interpreted as either. This section is intended to clarify that the
scope of applicability of the quoted text is a task. No R2T ordering
relationship -- either in generation at the target or in fulfilling
at the initiator -- across tasks is implied. That is, outstanding
R2Ts within a task MUST be fulfilled by the initiator in the order in
which they were received on a connection.
3.3. Model Assumptions for Response Ordering
Whenever an iSCSI session is composed of multiple connections, the
Response PDUs (task responses or TMF responses) originating in the
target SCSI layer are distributed onto the multiple connections by
the target iSCSI layer according to iSCSI connection allegiance
rules. This process generally may not preserve the ordering of the
responses by the time they are delivered to the initiator SCSI layer.
Since ordering is not expected across SCSI responses anyway, this
approach works fine in the general case. However, to address the
special cases where some ordering is desired by the SCSI layer, the
following "Response Fence" semantics are defined with respect to
handling SCSI response messages as they are handed off from the SCSI
protocol layer to the iSCSI layer.

3.3.1. Model Description
The target SCSI protocol layer hands off the SCSI response messages
to the target iSCSI layer by invoking the "Send Command Complete"
protocol data service ([SAM2], clause 5.4.2) and "Task Management
Function Executed" ([SAM2], clause 6.9) service. On receiving the
SCSI response message, the iSCSI layer exhibits the Response Fence
behavior for certain SCSI response messages (Section 3.3.3 describes
the specific instances where the semantics must be realized).
Whenever the Response Fence behavior is required for a SCSI response
message, the target iSCSI layer MUST ensure that the following
conditions are met in delivering the response message to the
initiator iSCSI layer:
(1) Response with Response Fence MUST be delivered chronologically
after all the "preceding" responses on the I_T_L nexus, if the
preceding responses are delivered at all, to the initiator iSCSI
layer.
(2) Response with Response Fence MUST be delivered chronologically
prior to all the "following" responses on the I_T_L nexus.
The "preceding" and "following" notions refer to the order of handoff
of a response message from the target SCSI protocol layer to the
target iSCSI layer.
3.3.2. iSCSI Semantics with the Interface Model
Whenever the TaskReporting key (Section 9.1) is negotiated to
ResponseFence or FastAbort for an iSCSI session and the Response
Fence behavior is required for a SCSI response message, the target
iSCSI layer MUST perform the actions described in this section for
that session.
a) If it is a single-connection session, no special processing is
required. The standard SCSI Response PDU build and dispatch
process happens.
b) If it is a multi-connection session, the target iSCSI layer
takes note of the last-sent and unacknowledged StatSN on each
of the connections in the iSCSI session, and waits for an
acknowledgement (NOP-In PDUs MAY be used to solicit
acknowledgements as needed in order to accelerate this process)
of each such StatSN to clear the fence. The SCSI response
requiring Response Fence behavior MUST NOT be sent to the
initiator before acknowledgements are received for each of the
unacknowledged StatSNs.

c) The target iSCSI layer must wait for an acknowledgement of the
SCSI Response PDU that carried the SCSI response requiring the
Response Fence behavior. The fence MUST be considered cleared
only after receiving the acknowledgement.
d) All further status processing for the LU is resumed only after
clearing the fence. If any new responses for the I_T_L nexus
are received from the SCSI layer before the fence is cleared,
those Response PDUs MUST be held and queued at the iSCSI layer
until the fence is cleared.
3.3.3. Current List of Fenced Response Use Cases
This section lists the fenced response use cases that iSCSI
implementations MUST comply with. However, this is not an exhaustive
enumeration. It is expected that as SCSI protocol specifications
evolve, the specifications will specify when response fencing is
required on a case-by-case basis.
Whenever the TaskReporting key (Section 9.1) is negotiated to
ResponseFence or FastAbort for an iSCSI session, the target iSCSI
layer MUST assume that the Response Fence is required for the
following SCSI completion messages:
1. The first completion message carrying the UA after the multi-
task abort on issuing and third-party sessions. See Section
4.1.1 for related TMF discussion.
2. The TMF Response carrying the multi-task TMF Response on the
issuing session.
3. The completion message indicating ACA establishment on the
issuing session.
4. The first completion message carrying the ACA ACTIVE status
after ACA establishment on issuing and third-party sessions.
5. The TMF Response carrying the Clear ACA response on the issuing
session.
6. The response to a PERSISTENT RESERVE OUT/PREEMPT AND ABORT
command.
Note: Due to the absence of ACA-related fencing requirements in
[RFC3720], initiator implementations SHOULD NOT use ACA on multi-
connection iSCSI sessions to targets complying only with [RFC3720].
Initiators that want to employ ACA on multi-connection iSCSI sessions

SHOULD first assess response-fencing behavior via negotiating for
ResponseFence or FastAbort values for the TaskReporting (Section 9.1)
key.
4. Task Management
4.1. Requests Affecting Multiple Tasks
This section clarifies and updates the original text in Section
10.6.2 of [RFC3720]. The clarified semantics (Section 4.1.2) are a
superset of the protocol behavior required in the original text and
all iSCSI implementations MUST support the new behavior. The updated
semantics (Section 4.1.3) on the other hand are mandatory only when
the new key TaskReporting (Section 9.1) is negotiated to "FastAbort".
4.1.1. Scope of Affected Tasks
This section defines the notion of "affected tasks" in multi-task
abort scenarios. Scope definitions in this section apply to both the
clarified protocol behavior (Section 4.1.2) and the updated protocol
behavior (Section 4.1.3).
ABORT TASK SET: All outstanding tasks for the I_T_L nexus
identified by the LUN field in the ABORT TASK SET TMF Request
PDU.
CLEAR TASK SET: All outstanding tasks in the task set for the LU
identified by the LUN field in the CLEAR TASK SET TMF Request
PDU. See [SPC3] for the definition of a "task set".
LOGICAL UNIT RESET: All outstanding tasks from all initiators for
the LU identified by the LUN field in the LOGICAL UNIT RESET
Request PDU.
TARGET WARM RESET/TARGET COLD RESET: All outstanding tasks from
all initiators across all LUs to which the TMF-issuing session
has access on the SCSI target device hosting the iSCSI session.
Usage: An "ABORT TASK SET TMF Request PDU" in the preceding text is
an iSCSI TMF Request PDU with the "Function" field set to "ABORT TASK
SET" as defined in [RFC3720]. Similar usage is employed for other
scope descriptions.

4.1.2. Clarified Multi-Task Abort Semantics
All iSCSI implementations MUST support the protocol behavior defined
in this section as the default behavior. The execution of ABORT TASK
SET, CLEAR TASK SET, LOGICAL UNIT RESET, TARGET WARM RESET, and
TARGET COLD RESET TMF Requests consists of the following sequence of
actions in the specified order on the specified party.
The initiator iSCSI layer:
a. MUST continue to respond to each TTT received for the affected
tasks.
b. SHOULD process any responses received for affected tasks in the
normal fashion. This is acceptable because the responses are
guaranteed to have been sent prior to the TMF response.
c. SHOULD receive the TMF Response concluding all the tasks in the
set of affected tasks unless the initiator has done something
(e.g., LU reset, connection drop) that may prevent the TMF
Response from being sent or received. The initiator MUST thus
conclude all affected tasks as part of this step in either
case, and MUST discard any TMF Response received after the
affected tasks are concluded.
The target iSCSI layer:
a. MUST wait for responses on currently valid target-transfer tags
of the affected tasks from the issuing initiator. MAY wait for
responses on currently valid target-transfer tags of the
affected tasks from third-party initiators.
b. MUST wait (concurrent with the wait in Step a) for all commands
of the affected tasks to be received based on the CmdSN
ordering. SHOULD NOT wait for new commands on third-party
affected sessions -- only the instantiated tasks have to be
considered for the purpose of determining the affected tasks.
In the case of target-scoped requests (i.e., TARGET WARM RESET
and TARGET COLD RESET), all of the commands that are not yet
received on the issuing session in the command stream however
can be considered to have been received with no command waiting
period -- i.e., the entire CmdSN space up to the CmdSN of the
task management function can be "plugged".
c. MUST propagate the TMF request to and receive the response from
the target SCSI layer.

d. MUST provide the Response Fence behavior for the TMF Response
on the issuing session as specified in Section 3.3.2.
e. MUST provide the Response Fence behavior on the first post-TMF
Response on third-party sessions as specified in Section 3.3.2.
If some tasks originate from non-iSCSI I_T_L nexuses, then the
means by which the target ensures that all affected tasks have
returned their status to the initiator are defined by the
specific non-iSCSI transport protocol(s).
Technically, the TMF servicing is complete in Step d. Data transfers
corresponding to terminated tasks may however still be in progress on
third-party iSCSI sessions even at the end of Step e. The TMF
Response MUST NOT be sent by the target iSCSI layer before the end of
Step d, and MAY be sent at the end of Step d despite these
outstanding data transfers until after Step e.
4.1.3. Updated Multi-Task Abort Semantics
Protocol behavior defined in this section MUST be implemented by all
iSCSI implementations complying with this document. Protocol
behavior defined in this section MUST be exhibited by iSCSI
implementations on an iSCSI session when they negotiate the
TaskReporting (Section 9.1) key to "FastAbort" on that session. The
execution of ABORT TASK SET, CLEAR TASK SET, LOGICAL UNIT RESET,
TARGET WARM RESET, and TARGET COLD RESET TMF Requests consists of the
following sequence of actions in the specified order on the specified
party.
The initiator iSCSI layer:
a. MUST NOT send any more Data-Out PDUs for affected tasks on the
issuing connection of the issuing iSCSI session once the TMF is
sent to the target.
b. SHOULD process any responses received for affected tasks in the
normal fashion. This is acceptable because the responses are
guaranteed to have been sent prior to the TMF response.
c. MUST respond to each Async Message PDU with AsyncEvent=5 as
defined in Section 8.1.
d. MUST treat the TMF response as terminating all affected tasks
for which responses have not been received, and MUST discard
any responses for affected tasks received after the TMF
response is passed to the SCSI layer (although the semantics

defined in this section ensure that such an out-of-order
scenario will never happen with a compliant target
implementation).
The target iSCSI layer:
a. MUST wait for all commands of the affected tasks to be received
based on the CmdSN ordering on the issuing session. SHOULD NOT
wait for new commands on third-party affected sessions -- only
the instantiated tasks have to be considered for the purpose of
determining the affected tasks. In the case of target-scoped
requests (i.e., TARGET WARM RESET and TARGET COLD RESET), all
the commands that are not yet received on the issuing session
in the command stream can be considered to have been received
with no command waiting period -- i.e., the entire CmdSN space
up to the CmdSN of the task management function can be
"plugged".
b. MUST propagate the TMF request to and receive the response from
the target SCSI layer.
c. MUST leave all active "affected TTTs" (i.e., active TTTs
associated with affected tasks) valid.
d. MUST send an Asynchronous Message PDU with AsyncEvent=5
(Section 8.1) on:
i) each connection of each third-party session to which at
least one affected task is allegiant if
TaskReporting=FastAbort is operational on that third-party
session, and
ii) each connection except the issuing connection of the
issuing session that has at least one allegiant affected
task.
If there are multiple affected LUs (say, due to a target reset),
then one Async Message PDU MUST be sent for each such LU on each
connection that has at least one allegiant affected task. The LUN
field in the Asynchronous Message PDU MUST be set to match the LUN
for each such LU.
e. MUST address the Response Fence flag on the TMF Response on the
issuing session as defined in Section 3.3.2.
f. MUST address the Response Fence flag on the first post-TMF
Response on third-party sessions as defined in Section 3.3.2.
If some tasks originate from non-iSCSI I_T_L nexuses, then the

means by which the target ensures that all affected tasks have
returned their status to the initiator are defined by the
specific non-iSCSI transport protocol(s).
g. MUST free up the affected TTTs (and STags, if applicable) and
the corresponding buffers, if any, once it receives each
associated NOP-Out acknowledgement that the initiator generated
in response to each Async Message.
Technically, the TMF servicing is complete in Step e. Data transfers
corresponding to terminated tasks may however still be in progress
even at the end of Step f. A TMF Response MUST NOT be sent by the
target iSCSI layer before the end of Step e, and MAY be sent at the
end of Step e despite these outstanding Data transfers until Step g.
Step g specifies an event to free up any such resources that may have
been reserved to support outstanding data transfers.
4.1.3.1. Clearing Effects Update
Appendix F.1 of [RFC3720] specifies the clearing effects of target
and LU resets on "Incomplete TTTs" as "Y". This meant that a target
warm reset or a target cold reset or an LU reset would clear the
active TTTs upon completion. However, the TaskReporting=FastAbort
(Section 9.1) semantics defined by this section do not guarantee that
the active TTTs are cleared by the end of the reset operations. In
fact, the new semantics are designed to allow clearing the TTTs in a
"lazy" fashion after the TMF Response is delivered. Thus, when
TaskReporting=FastAbort is operational on a session, the clearing
effects of reset operations on "Incomplete TTTs" is "N".
4.1.4. Affected Tasks Shared across RFC 3720 and FastAbort Sessions
If an iSCSI target implementation is capable of supporting
TaskReporting=FastAbort functionality (Section 9.1), it may end up in
a situation where some sessions have TaskReporting=RFC3720
operational (RFC 3720 sessions) while some other sessions have
TaskReporting=FastAbort operational (FastAbort sessions) even while
accessing a shared set of affected tasks (Section 4.1.1).
If the issuing session is an RFC 3720 session, the iSCSI target
implementation is FastAbort-capable, and the third-party affected
session is a FastAbort session, the following behavior SHOULD be
exhibited by the iSCSI target layer:
a. Between Steps c and d of the target behavior in Section 4.1.2,
send an Asynchronous Message PDU with AsyncEvent=5 (Section
8.1) on each connection of each third-party session to which at
least one affected task is allegiant. If there are multiple

affected LUs, then send one Async Message PDU for each such LU
on each connection that has at least one allegiant affected
task. When sent, the LUN field in the Asynchronous Message PDU
MUST be set to match the LUN for each such LU.
b. After Step e of the target behavior in Section 4.1.2, free up
the affected TTTs (and STags, if applicable) and the
corresponding buffers, if any, once each associated NOP-Out
acknowledgement is received that the third-party initiator
generated in response to each Async Message sent in Step a.
If the issuing session is a FastAbort session, the iSCSI target
implementation is FastAbort-capable, and the third-party affected
session is an RFC 3720 session, the following behavior MUST be
exhibited by the iSCSI target layer: Asynchronous Message PDUs MUST
NOT be sent on the third-party session to prompt the FastAbort
behavior.
If the third-party affected session is a FastAbort session and the
issuing session is a FastAbort session, the initiator in the third-
party role MUST respond to each Async Message PDU with AsyncEvent=5
as defined in Section 8.1. Note that an initiator MAY thus receive
these Async Messages on a third-party affected session even if the
session is a single-connection session.
4.1.5. Implementation Considerations
Both in clarified semantics (Section 4.1.2) and updated semantics
(Section 4.1.3), there may be outstanding data transfers even after
the TMF completion is reported on the issuing session. In the case
of iSCSI/iSER [iSER], these would be tagged data transfers for STags
not owned by any active tasks. Whether or not real buffers support
these data transfers is implementation-dependent. However, the data
transfers logically MUST be silently discarded by the target iSCSI
layer in all cases. A target MAY, on an implementation-defined
internal timeout, also choose to drop the connections on which it did
not receive the expected Data-Out sequences (Section 4.1.2) or NOP-
Out acknowledgements (Section 4.1.3) so as to reclaim the associated
buffer, STag, and TTT resources as appropriate.
4.1.6. Rationale behind the New Semantics
There are fundamentally three basic objectives behind the semantics
specified in Sections 4.1.2 and 4.1.3.
1. Maintaining an ordered command flow I_T nexus abstraction to
the target SCSI layer even with multi-connection sessions.

o Target iSCSI processing of a TMF request must maintain the
single flow illusion. Target behavior in Step b of Section
4.1.2 and Step a of Section 4.1.3 correspond to this
objective.
2. Maintaining a single ordered response flow I_T nexus
abstraction to the initiator SCSI layer even with multi-
connection sessions when one response (i.e., TMF response)
could imply the status of other unfinished tasks from the
initiator's perspective.
o The target must ensure that the initiator does not see "old"
task responses (that were placed on the wire chronologically
earlier than the TMF Response) after seeing the TMF
response. The target behavior in Step d of Section 4.1.2
and Step e of Section 4.1.3 correspond to this objective.
o Whenever the result of a TMF action is visible across
multiple I_T_L nexuses, [SAM2] requires the SCSI device
server to trigger a UA on each of the other I_T_L nexuses.
Once an initiator is notified of such an UA, the application
client on the receiving initiator is required to clear its
task state (clause 5.5 in [SAM2]) for the affected tasks.
It would thus be inappropriate to deliver a SCSI Response
for a task after the task state is cleared on the initiator,
i.e., after the UA is notified. The UA notification
contained in the first SCSI Response PDU on each affected
Third-party I_T_L nexus after the TMF action thus MUST NOT
pass the affected task responses on any of the iSCSI
sessions accessing the LU. The target behavior in Step e of
Section 4.1.2 and Step f of Section 4.1.3 correspond to this
objective.
3. Draining all active TTTs corresponding to affected tasks in a
deterministic fashion.
o Data-Out PDUs with stale TTTs arriving after the tasks are
terminated can create a buffer management problem even for
traditional iSCSI implementations, and is fatal for the
connection for iSCSI/iSER implementations. Either the
termination of affected tasks should be postponed until the
TTTs are retired (as in Step a of Section 4.1.2), or the
TTTs and the buffers should stay allocated beyond task
termination to be deterministically freed up later (as in
Steps c and g of Section 4.1.3).
The only other notable optimization is the plugging. If all tasks on
an I_T nexus will be aborted anyway (as with a target reset), there

is no need to wait to receive all commands to plug the CmdSN holes.
The target iSCSI layer can simply plug all missing CmdSN slots and
move on with TMF processing. The first objective (maintaining a
single ordered command flow) is still met with this optimization
because the target SCSI layer only sees ordered commands.
5. Discovery Semantics
5.1. Error Recovery for Discovery Sessions
The negotiation of the key ErrorRecoveryLevel is not required for
Discovery sessions -- i.e., for sessions that negotiated
"SessionType=Discovery" -- because the default value of 0 is
necessary and sufficient for Discovery sessions. It is however
possible that some legacy iSCSI implementations might attempt to
negotiate the ErrorRecoveryLevel key on Discovery sessions. When
such a negotiation attempt is made by the remote side, a compliant
iSCSI implementation MUST propose a value of 0 (zero) in response.
The operational ErrorRecoveryLevel for Discovery sessions thus MUST
be 0. This naturally follows from the functionality constraints that
[RFC3720] imposes on Discovery sessions.
5.2. Reinstatement Semantics of Discovery Sessions
Discovery sessions are intended to be relatively short-lived.
Initiators are not expected to establish multiple Discovery sessions
to the same iSCSI Network Portal (see [RFC3720]). An initiator may
use the same iSCSI Initiator Name and ISID when establishing
different unique sessions with different targets and/or different
portal groups. This behavior is discussed in Section 9.1.1 of
[RFC3720] and is, in fact, encouraged as conservative reuse of ISIDs.
The ISID RULE in [RFC3720] states that there must not be more than
one session with a matching 4-tuple: <InitiatorName, ISID,
TargetName, TargetPortalGroupTag>. While the spirit of the ISID RULE
applies to Discovery sessions the same as it does for Normal
sessions, note that some Discovery sessions differ from the Normal
sessions in two important aspects:
Because [RFC3720] allows a Discovery session to be established
without specifying a TargetName key in the Login Request PDU (let
us call such a session an "Unnamed" Discovery session), there is
no Target Node context to enforce the ISID RULE.
Portal Groups are defined only in the context of a Target Node.
When the TargetName key is NULL-valued (i.e., not specified), the
TargetPortalGroupTag thus cannot be ascertained to enforce the
ISID RULE.

The following sections describe the two scenarios -- Named Discovery
sessions and Unnamed Discovery sessions -- separately.
5.2.1. Unnamed Discovery Sessions
For Unnamed Discovery sessions, neither the TargetName nor the
TargetPortalGroupTag is available to the targets in order to enforce
the ISID RULE. So the following rule applies.
UNNAMED ISID RULE: Targets MUST enforce the uniqueness of the
following 4-tuple for Unnamed Discovery sessions: <InitiatorName,
ISID, NULL, TargetAddress>. The following semantics are implied by
this uniqueness requirement.
Targets SHOULD allow concurrent establishment of one Discovery
session with each of its Network Portals by the same initiator port
with a given iSCSI Node Name and an ISID. Each of the concurrent
Discovery sessions, if established by the same initiator port to
other Network Portals, MUST be treated as independent sessions --
i.e., one session MUST NOT reinstate the other.
A new Unnamed Discovery session that has a matching <InitiatorName,
ISID, NULL, TargetAddress> to an existing Discovery session MUST
reinstate the existing Unnamed Discovery session. Note thus that
only an Unnamed Discovery session may reinstate an Unnamed Discovery
session.
5.2.2. Named Discovery Sessions
For a Named Discovery session, the TargetName key is specified by the
initiator and thus the target can unambiguously ascertain the
TargetPortalGroupTag as well. Since all the four elements of the 4-
tuple are known, the ISID RULE MUST be enforced by targets with no
changes from [RFC3720] semantics. A new session with a matching
<InitiatorName, ISID, TargetName, TargetPortalGroupTag> thus will
reinstate an existing session. Note in this case that any new iSCSI
session (Discovery or Normal) with the matching 4-tuple may reinstate
an existing Named Discovery iSCSI session.
5.3. Target PDUs during Discovery
Targets SHOULD NOT send any responses other than a Text Response and
Logout Response on a Discovery session, once in Full Feature Phase.
Implementation Note: A target may simply drop the connection in a
Discovery session when it would have requested a Logout via an Async
Message on Normal sessions.

6. Negotiation and Others
6.1. TPGT Values
[SAM2] and [SAM3] specifications incorrectly note in their
informative text that TPGT value should be non-zero, although
[RFC3720] allows the value of zero for TPGT. This section is to
clarify that a zero value is expressly allowed as a legal value for
TPGT. This discrepancy currently stands corrected in [SAM4].
6.2. SessionType Negotiation
During the Login Phase, the SessionType key is offered by the
initiator to choose the type of session it wants to create with the
target. The target may accept or reject the offer. Depending on the
type of the session, a target may decide on resources to allocate and
the security to enforce, etc. for the session. If the SessionType
key is thus going to be offered as "Discovery", it SHOULD be offered
in the initial Login request by the initiator.
6.3. Understanding NotUnderstood
[RFC3720] defines NotUnderstood as a valid answer during a
negotiation text key exchange between two iSCSI nodes. NotUnderstood
has the reserved meaning that the sending side did not understand the
proposed key semantics. This section seeks to clarify that
NotUnderstood is a valid answer for both declarative and negotiated
keys. The general iSCSI philosophy is that comprehension precedes
processing for any iSCSI key. A proposer of an iSCSI key, negotiated
or declarative, in a text key exchange MUST thus be able to properly
handle a NotUnderstood response.
The proper way to handle a NotUnderstood response depends on where
the key is specified and whether the key is declarative vs.
negotiated. All keys defined in [RFC3720] MUST be supported by all
compliant implementations; a NotUnderstood answer on any of the
[RFC3720] keys therefore MUST be considered a protocol error and
handled accordingly. For all other later keys, a NotUnderstood
answer concludes the negotiation for a negotiated key whereas for a
declarative key, a NotUnderstood answer simply informs the declarer
of a lack of comprehension by the receiver.
In either case, a NotUnderstood answer always requires that the
protocol behavior associated with that key not be used within the
scope of the key (connection/session) by either side.

6.4. Outstanding Negotiation Exchanges
There was some uncertainty around the number of outstanding Login
Response PDUs on a connection. [RFC3720] offers the analogy of SCSI
linked commands to Login and Text negotiations in Sections 5.3 and
10.10.3, respectively, but does not make it fully explicit. This
section is to offer a clarification in this regard.
There MUST NOT be more than one outstanding Login Request, Login
Response, Text Request, or Text Response PDU on an iSCSI connection.
An outstanding PDU in this context is one that has not been
acknowledged by the remote iSCSI side.
7. iSCSI Error Handling and Recovery
7.1. ITT
An ITT value of 0xffffffff is reserved and MUST NOT be assigned for a
task by the initiator. The only instance in which it may be seen on
the wire is in a target-initiated NOP-In PDU (and in the initiator
response to that PDU, if necessary). Section 10.19 in [RFC3720]
mentions this in passing but is noted here again to make it obvious
since the semantics apply to the initiators in general.
7.2. Format Errors
Section 6.6 of [RFC3720] discusses format error handling. This
section elaborates on the "inconsistent" PDU field contents noted in
[RFC3720].
All initiator-detected PDU construction errors MUST be considered as
format errors. Some examples of such errors are:
- NOP-In with a valid TTT but an invalid LUN
- NOP-In with a valid ITT (i.e., a NOP-In response) and also a
valid TTT
- SCSI Response PDU with Status=CHECK CONDITION, but
DataSegmentLength = 0
7.3. Digest Errors
Section 6.7 of [RFC3720] discusses digest error handling. It states
that "No further action is necessary for initiators if the discarded
PDU is an unsolicited PDU (e.g., Async, Reject)" on detecting a
payload digest error. This is incorrect.

An Asynchronous Message PDU or a Reject PDU carries the next StatSN
value on an iSCSI connection, advancing the StatSN. When an
initiator discards one of these PDUs due to a payload digest error,
the entire PDU including the header MUST be discarded. Consequently,
the initiator MUST treat the exception like a loss of any other
solicited response PDU -- i.e., it MUST use one of the following
options noted in [RFC3720]:
a) Request PDU retransmission with a status SNACK.
b) Logout the connection for recovery and continue the tasks on a
different connection instance.
c) Logout to close the connection (abort all the commands
associated with the connection).
7.4. Message Error Checking
There has been some uncertainty on the extent to which incoming
messages have to be checked for protocol errors, beyond what is
strictly required for processing the inbound message. This section
addresses this question.
Unless [RFC3720] or this document requires it, an iSCSI
implementation is not required to do an exhaustive protocol
conformance check on an incoming iSCSI PDU. The iSCSI implementation
especially is not required to double-check the remote iSCSI
implementation's conformance to protocol requirements.
8. iSCSI PDUs
8.1. Asynchronous Message
This section defines additional semantics for the Asynchronous
Message PDU defined in Section 10.9 of [RFC3720] using the same
conventions.
The following new legal value for the AsyncEvent is defined:
5: all active tasks for LU with a matching LUN field in the Async
Message PDU are being terminated.
The receiving initiator iSCSI layer MUST respond to this Message by
taking the following steps in order.
i) Stop Data-Out transfers on that connection for all active TTTs
for the affected LUN quoted in the Async Message PDU.

ii) Acknowledge the StatSN of the Async Message PDU via a NOP-Out
PDU with ITT=0xffffffff (i.e., non-ping flavor), while copying
the LUN field from the Async Message to NOP-Out.
The new AsyncEvent defined in this section however MUST NOT be used
on an iSCSI session unless the new TaskReporting text key defined in
Section 9.1 was negotiated to FastAbort on the session.
8.2. Reject
Section 10.17.1 of [RFC3720] specifies the Reject reason code of 0x0b
with an explanation of "Negotiation Reset". At this point, we do not
see any legitimate iSCSI protocol use case for using this reason
code. Thus, reason code 0x0b MUST be considered as deprecated and
MUST NOT be sent by implementations that comply with the requirements
of this document. An implementation receiving reason code 0x0b MUST
treat it as a negotiation failure that terminates the Login Phase and
the TCP connection, as specified in Section 6.10 of [RFC3720].
Section 5.4 of [RFC3720] states:
Neither the initiator nor the target should attempt to declare or
negotiate a parameter more than once during any negotiation
sequence without an intervening operational parameter negotiation
reset, except for responses to specific keys that explicitly allow
repeated key declarations (e.g., TargetAddress).
The deprecation of reason code 0x0b eliminates the possibility of an
operational parameter negotiation reset, causing the phrase "without
an intervening operational parameter negotiation reset" in [RFC3720]
to refer to an impossible event. The quoted phrase SHOULD be ignored
by receivers that handle reason code 0x0b in the manner specified in
this section.
9. Login/Text Operational Text Keys
This section follows the same conventions as Section 12 of [RFC3720].
9.1. TaskReporting
Use: LO
Senders: Initiator and Target
Scope: SW
Irrelevant when: SessionType=Discovery
TaskReporting=<list-of-values>

Default is RFC3720.
Result function is AND.
This key is used to negotiate the task completion reporting semantics
from the SCSI target. The following table describes the semantics
that an iSCSI target MUST support for respective negotiated key
values. Whenever this key is negotiated, at least the RFC3720 and
ResponseFence values MUST be offered as options by the negotiation
originator.
+--------------+------------------------------------------+
| Name | Description |
+--------------+------------------------------------------+
| RFC3720 | RFC 3720-compliant semantics. Response |
| | fencing is not guaranteed and fast |
| | completion of multi-task aborting is not |
| | supported |
+--------------+------------------------------------------+
| ResponseFence| Response Fence (Section 3.3.1) semantics |
| | MUST be supported in reporting task |
| | completions |
+--------------+------------------------------------------+
| FastAbort | Updated fast multi-task abort semantics |
| | defined in Section 4.1.3 MUST be |
| | supported. Support for Response Fence is|
| | implied -- i.e., Section 3.3.1 semantics |
| | MUST be supported as well |
+--------------+------------------------------------------+
When TaskReporting is not negotiated to FastAbort, the [RFC3720] TMF
semantics as clarified in Section 4.1.2 MUST be used.
10. Security Considerations
This document does not introduce any new security considerations
other than those already noted in [RFC3720]. Consequently, all the
iSCSI-related security text in [RFC3723] is also directly applicable
to this document.

11. IANA Considerations
11.1. iSCSI-Related IANA Registries
This document creates the following iSCSI-related registries for IANA
to manage.
1. iSCSI Opcodes
2. iSCSI Login/Text Keys
3. iSCSI Asynchronous Events
4. iSCSI Task Management Function Codes
5. iSCSI Login Response Status Codes
6. iSCSI Reject Reason Codes
7. iSER Opcodes
Each of the following sections describes a registry, its sub-
registries if any, and their administration policies in more detail.
IANA has registered what this document calls the main "registries" as
sub-registries of a larger iSCSI registry. However, wherever
registry-to-sub-registry relationships are specified by this
document, they have been preserved intact.
[RFC3720] specifies three iSCSI-related registries -- extended key,
authentication methods, and digests. This document recommends that
these registries be published together with the registries defined by
this document. Further, this document recommends that the three
[RFC3720] registries be listed in between items 6 and 7 in the
registry list above.
11.2. iSCSI Opcodes
Name of the registry: "iSCSI Opcodes"
Namespace details: Numerical values that can fit in one octet with
the most significant two bits (bits 0 and 1) already designated by
[RFC3720], bit 0 being reserved and bit 1 for immediate delivery.
Bit 2 is designated to identify the originator of the opcode. Bit 2
= 0 for initiator and Bit 2 = 1 for target.

Information that must be provided to assign a new value: An IESG-
approved standards-track specification defining the semantics and
interoperability requirements of the proposed new value and the
fields to be recorded in the registry.
Allocation request guidance to requesters:
1) If the initiator opcode and target opcode used to identify the
request and response of a new type of protocol operation are
requested, assign the same lower five bits (i.e., Bit 3 through
Bit 7) for both opcodes, e.g., 0x13 and 0x33.
2) If only the initiator opcode or target opcode is requested to
identify a one-way protocol message (i.e., request without a
response or a "response" without a request), assign an unused
number from the appropriate category (i.e., Bit 2 set to 0 or 1
for initiator category or target category) and add the other
pair member (i.e., same opcode with Bit 2 set to 1 or 0,
respectively) to "no opcode pair is available" list.
3) If there are no other opcodes available in the appropriate
"Reserved to IANA" list to assign on a request for a new opcode
except the reserved opcodes in the "no opcode pair is
available" list, allocate the opcode from the appropriate
category (initiator or target) of the "no opcode pair is
available" list.
Fields to record in the registry: Assigned value, Who can originate
(Initiator or Target), Operation Name, and its associated RFC
reference.
Initial registry contents:
0x00, Initiator, NOP-Out, [RFC3720]
0x01, Initiator, SCSI Command, [RFC3720]
0x02, Initiator, SCSI Task Management function request, [RFC3720]
0x03, Initiator, Login Request, [RFC3720]
0x04, Initiator, Text Request, [RFC3720]
0x05, Initiator, SCSI Data-Out, [RFC3720]
0x06, Initiator, Logout Request, [RFC3720]
0x10, Initiator, SNACK Request, [RFC3720]

Information that must be provided to assign a new value: An IESG-
approved standards-track specification defining the semantics and
interoperability requirements of the proposed new Key per [RFC3720]
key specification template and the fields to be recorded in the
registry.
Assignment policy:
If the requested Key name is not already assigned and is roughly
representative of its proposed semantics, it may be assigned to the
requester.
Given the arbitrary nature of text strings, there is no maximum
reserved by IANA for assignment in this registry.
Fields to record in the registry: Assigned Key Name and its
associated RFC reference.
Initial registry contents:
AuthMethod, [RFC3720]
HeaderDigest, [RFC3720]
DataDigest, [RFC3720]
MaxConnections, [RFC3720]
SendTargets, [RFC3720]
TargetName, [RFC3720]
InitiatorName, [RFC3720]
TargetAlias, [RFC3720]
InitiatorAlias, [RFC3720]
TargetAddress, [RFC3720]
TargetPortalGroupTag, [RFC3720]
InitialR2T, [RFC3720]
ImmediateData, [RFC3720]
MaxRecvDataSegmentLength, [RFC3720]

MaxBurstLength, [RFC3720]
FirstBurstLength, [RFC3720]
DefaultTime2Wait, [RFC3720]
DefaultTime2Retain, [RFC3720]
MaxOutstandingR2T, [RFC3720]
DataPDUInOrder, [RFC3720]
DataSequenceInOrder, [RFC3720]
ErrorRecoveryLevel, [RFC3720]
SessionType, [RFC3720]
RDMAExtensions, [iSER]
TargetRecvDataSegmentLength, [iSER]
InitiatorRecvDataSegmentLength, [iSER]
MaxOutstandingUnexpectedPDUs, [iSER]
TaskReporting, this document
Allocation Policy:
Standards Action ([IANA])
11.4. iSCSI Asynchronous Events
Name of the registry: "iSCSI Asynchronous Events"
Namespace details: Numerical values that can fit in one octet.
Information that must be provided to assign a new value: An IESG-
approved standards-track specification defining the semantics and
interoperability requirements of the proposed new Event and the
fields to be recorded in the registry.
Assignment policy:
If the requested value is not already assigned, it may be assigned to
the requester.

6-247: range reserved by IANA for assignment in this registry.
Fields to record in the registry: Assigned Event number, Description
and its associated RFC reference.
Initial registry contents:
0, SCSI Async Event, [RFC3720]
1, Logout Request, [RFC3720]
2, Connection drop notification, [RFC3720]
3, Session drop notification, [RFC3720]
4, Negotiation Request, [RFC3720]
5, Task termination, this document
248-254, Vendor-unique, this document
255, Vendor-unique, [RFC3720]
Allocation Policy:
Standards Action ([IANA])
11.5. iSCSI Task Management Function Codes
Name of the registry: "iSCSI TMF Codes"
Namespace details: Numerical values that can fit in 7 bits.
Information that must be provided to assign a new value: An IESG-
approved standards-track specification defining the semantics and
interoperability requirements of the proposed new Code and the fields
to be recorded in the registry.
Assignment policy:
If the requested value is not already assigned, it may be assigned to
the requester.
9-127: range reserved by IANA for assignment in this registry.
Fields to record in the registry: Assigned Code, Description, and its
associated RFC reference.

Initial registry contents:
1, ABORT TASK, [RFC3720]
2, ABORT TASK SET, [RFC3720]
3, CLEAR ACA, [RFC3720]
4, CLEAR TASK SET, [RFC3720]
5, LOGICAL UNIT RESET, [RFC3720]
6, TARGET WARM RESET, [RFC3720]
7, TARGET COLD RESET, [RFC3720]
8, TASK REASSIGN, [RFC3720]
Allocation Policy:
Standards Action ([IANA])
11.6. iSCSI Login Response Status Codes
Name of the registry: "iSCSI Login Response Status Codes"
Namespace details: Numerical values; Status-Class (one octet),
Status-Detail (one octet) for each possible value of Status-Class
except for Vendor-Unique Status-Class (0x0f).
Information that must be provided to assign a new value: An IESG-
approved specification defining the semantics and interoperability
requirements of the proposed new Code, its Status-Class affiliation
(only if a new Status-Detail value is being proposed for a Status-
Class), Status-Class definition (only if a new Status-Class is being
proposed), and the fields to be recorded in the registry.
Assignment policy:
If the requested value is not already assigned, it may be assigned to
the requester.
4-14 and 16-255: range reserved by IANA for assignment in this
registry.
Fields to record in the Status-Class main registry: Assigned Code,
Description, and its associated RFC reference.

0x02, 0x08, Can't include in session, [RFC3720]
0x02, 0x09, Unsupported session type, [RFC3720]
0x02, 0x0a, Non-existent session, [RFC3720]
0x02, 0x0b, Invalid during login, [RFC3720]
12-255: range reserved by IANA for assignment in Status-Class=2 sub-
registry.
Initial sub-registry contents (Status-Detail for Status-Class=0x03):
0x03, 0x00, Target error, [RFC3720]
0x03, 0x01, Service unavailable, [RFC3720]
0x03, 0x02, Out of resources, [RFC3720]
3-255: range reserved by IANA for assignment in Status-Class=3 sub-
registry.
Allocation Policy:
Standards Action ([IANA])
11.7. iSCSI Reject Reason Codes
Name of the registry: "iSCSI Reject Reason Codes"
Namespace details: Numerical values that can fit in a single octet.
Information that must be provided to assign a new value: A published
specification defining the semantics and interoperability
requirements of the proposed new Code and the fields to be recorded
in the registry.
Assignment policy:
If the requested value is not already assigned, it may be assigned to
the requester.
13-255: range reserved by IANA for assignment in this registry.
Fields to record in the registry: Assigned Code, Description, and its
associated RFC reference.

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